This invention relates generally to a method and apparatus for testing and calibrating sensors, and more particularly to a method and apparatus for testing and calibrating capacitive sensors.
A need exists for a highly sensitive and selective detector capable of quantifying chemical, physical or biological presences or components in a monitored area. To be useful, such a detector should be small, rugged, inexpensive, selective, reversible and extremely sensitive. Applicants and/or the assignee of the invention (or its predecessors in interest) have invented other apparatus directed to this need; U.S. Pat. No. 5,719,324 to Thundat et al., U.S. Pat. No. 5,445,008 to Wachter et al. and U.S. Pat. No. 6,167,748 to Britton, Jr. et al. (Britton), which are all incorporated herein by reference.
The above references each disclose microcapacitive sensor technology which represents an electromechanical technique with broad applications in chemical, physical and biological detection. Depending on the dimensions of the microcapacitive sensor platform and the suspended element spring constant, deflections of these suspended elements can be detected with sub-angstrom precision using current techniques employed by atomic force microscopy (AMF) technology such as optical, piezoresistive, piezoelectric, capacitive, and electron tunneling.
Britton discloses arrayable, electronically read cantilever suspended elements which are selectively coated with certain chemicals to achieve a sensitivity to a specific physical, chemical or biological presence, e.g., relative humidity, mercury vapor, mercaptan, toluene, viscosity infrared and ultraviolet radiation, flow rate, lead in water, DNA hybridization, and antibody-antigen interaction. As the coatings react with the presence or component sensed, the resulting change in stress causes a deflection of the cantilever, which alters the spacing between the cantilever its associated lower xe2x80x9cpick-upxe2x80x9d plate. This spacing change is sensed as a change in capacitance, since capacitance of a parallel plate capacitor is inversely proportional to the plate separation distance. The change in capacitance is converted into a signal by a sensing circuit which is ultimately converted into a DC voltage output for further processing. An array of cantilevers with a variety of different coatings may be placed on a single chip in order to detect the simultaneous presence of multiple various chemical, physical or biological presences. As used herein, the process by which the presence of chemical, physical or biological presences is detected by a capacitive sensor is referred to as xe2x80x9cfield usexe2x80x9d of the capacitive sensor.
Microelectromechanical sensors (MEMS) are a new class of microfabricated structures which generally feature mechanically moving components located on a plurality of die of silicon or other suitable bulk substrate material. Only recently have MEMS arrays been demonstrated for microcapacitive sensor technology to permit detection of various physical, chemical or biological agents based on changes in capacitance of a cantilever sensor. Capacitive actuation or readout is preferred for these sensors because of relative ease of integration and low-power requirements. Low power requirements enable battery power supplies for certain applications.
One common problem with MEMS fabrication processes is the yield. For example, thin sacrificial layers, such as xcexc or sub-xcexc layers of SiO2 are commonly used to deposit the suspended layer (e.g. polysilicon) thereon. The sacrificial layer may then be removed to form the suspended element to complete the microcapacitive sensor. However, short gap distances common in these devices often result in incomplete removal of the sacrificial material in the gap. Another common yield problem is stiction, which occurs when two adjacent surfaces adhere. Various forms of contamination can also occur at the foundry, in subsequent processing, or in actual use. Methods are not currently available to diagnose yield problems such as those noted above during processing. Instead, these problems are typically only discovered at final assembly and test, after significant funds have typically been expended.
Another problem with MEMS sensors which use capacitive readouts is that their sensitivity can vary with residual stress. For example, the residual stress in polysilicon suspended elements used in surface-micromachined MEMS must be controlled so that warpage does not render the sensor unusable. Also, commonly applied coating layers generally result in stresses that can often significantly warp the normally thin suspended elements. Consequently, the gap between adjacent sensor capacitor plates can change affecting the magnitude of the mutual plate capacitance and as a result change the sensitivity of the sensor, the sensitivity being defined as signal output divided by sensory input.
If the gap distance were conveniently obtainable, microcapacitive sensor chips could be probed during processing, to determine whether to commit a given chip to costly assembly and post-assembly testing. In addition, during use, correction factors for the sensor""s sensitivity could be determined and applied, and preferably updated periodically during the operation of the sensor. This is important since stresses to the suspended element can change over time or with environmental factors. Accordingly, it is desirable to recalibrate a microcapacitive sensor to account for changes in the gap distance and stiffness of the suspended element which can change over time. Calibration is highly desirable and can be essential for accurate detection required for most MEMS sensor applications. Thus, a convenient method for directly or indirectly measuring the gap distance and the stiffness of suspended elements in capacitive MEMS sensors is needed. The method and apparatus should preferably be fast, capable of automation and be adapted for implementation as an automatic imbedded self-test and calibration feature for finished capacitive sensor devices.
A method for determining operational characteristics of capacitive sensors includes the steps of providing at least one capacitive sensor, the capacitive sensor having plates. The plates include a suspended element and a pick-up plate, the suspended element capable of displacement relative to the pick-up plate by application of a force, the displacement and the force related by a spring constant. An AC input signal is applied to one of the plates, while force is provided between the plates. The force is capable of variation. Based on measured output values, at least one operational characteristic of the capacitive sensor is determined. The AC input signal can be a voltage signal or a current signal.
The method can determine a distance (gap) between the plates in the determining step. The force can include an electrostatic force provided by applying a DC bias between the plates. By varying the amplitude of the DC bias and measuring resulting AC output signals, a spring constant for the suspended element can be determined.
The force can include a magnetic force. To produce the magnetic force, the suspended element and the pick-up plate can be magnetic. Magnetic plates can be provided from plates having magnetically polarizable material and/or electromagnets.
Electrostatic and magnetic forces can be combined. For example, an electrostatic attractive force can be combined with a repulsive magnetic force. In one embodiment, the magnetic force is made substantially equal in magnitude and opposite in direction to the electrostatic force. The method can include the step of varying an amplitude of the DC bias while a magnetic force is applied, and measuring at least one resulting output signal, wherein a spring constant for the suspended element can be determined during the determining step.
The method can include the step of storing the determined spring constant and the gap. The method can include the step of applying at least one of the spring constant and the gap to field measurements made by the capacitive sensor. The spring constant and the gap can be automatically updated during the field use, and preferably be updated continuously.
A method for operating capacitive sensors includes the steps of providing at least one capacitive sensor and at least one self-testing and calibration network. The capacitive sensor has plates, the plates including a suspended element and a pick-up plate. An AC input signal is applied to one of the plates, and force is provided between the plates, the force capable of variation which allows at least one operational characteristic of the capacitive sensor to be determined. The spring constant for the suspended element and a gap between the plates can be determined during the determining step. The method can include the step of storing the spring constant and the gap.
The spring constant and the gap can be updated automatically during field use of the capacitive sensor. Upon generation of a measurement signal responsive to the detection of the presence of at least one material, at least one of the spring constant and the gap can be applied to the measurement signal.
A method for determining gap distances between capacitive sensor plates includes the steps of providing at least one capacitive sensor, the capacitive sensor having plates, the plates including a suspended element and a pick-up plate. An AC input signal is applied for generating a displacement current across the capacitive sensor, the AC input signal applied to one of the plates. A resulting output signal is measured and a gap distance between the plates is determined from the resulting measured output signal. The method can include the step of storing the gap data. The gap can be automatically updated during field use of the capacitive sensor, or preferably continuously updated during field use.
The method can also include the steps of generating a measurement signal during field use responsive to the detection of the presence of at least one material, and applying the gap to the measurement signal. In this embodiment, the gap can be automatically updated during field use, preferably continuously updated during field use.
A method for determining at least one operational characteristic of capacitive sensors includes the steps of providing at least one capacitive sensor, the capacitive sensor having a suspended element and a pick-up plate. Force is provided between the plates to provide a stress to the suspended element, the force capable of variation. An optical beam is directed onto the suspended element to generate a reflected beam. The reflected beam is then detected. An AC input signal can be applied to one of the plates to electrically measure a gap between the plates. If an AC signal is used, the method can further include the step of optically determining the gap using the detected reflected optical beam, wherein the electrical gap measurement can be compared to the optically determined gap.
A capacitive sensor system includes at least one capacitive sensor, the capacitive sensor including a suspended element and a pick-up plate and at least one self-testing and calibration network. The self-testing and calibration network includes a processor, the self-testing and calibration network being switchably connected to the capacitive sensor. A structure for generation of force between the plates is included, the force capable of variation. The processor can be adapted for determining parameters including a gap distance between the plates and a spring constant of the suspended element. The system can also include a memory.
The processor can be adapted to automatically apply at least one of the spring constant and the gap to measurement signals resulting from the detection of the presence of at least one material obtained during field use of the capacitive sensor. The spring constant and the gap can be periodically updated during field use. The updating can be continuous.
The structure for generation of force can include a DC bias applied between the plates to create an electrostatic force between the plates. Alternatively, the structure for generation of force can include magnetic suspended elements and pick-up plates to create a magnetic force between the plates. Magnetic plates can be formed by using magnetically polarizable material and/or electromagnets. The magnetic force can be substantially equal in magnitude and opposite in direction to the electrostatic force.
The system can also include a bulk substrate material having as plurality of die, at least one capacitive sensor formed on at least one die. A package can be included for bonding die to the package, wherein the self-testing and calibration network can be contained in the package with at least one of the capacitive sensors. Preferably, the self-testing and calibration network and at least one capacitive sensor can be formed on a single die. The system can include at least one oscillator circuit for generating an AC signal from a DC input. The oscillator can be formed on a die along with the testing and calibration network, and at least one capacitive sensor.